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2. Determination of the Nickel, Cobalt, Iron, Zinc and Man-
ganese.—The nitrate from the bismuth, silver, cadmium and copper sul-
phides is collected in a flask, rendered slightly ammoniacal and treated with
3 c.c. of ammonium sulphide. The flask is filled with water and left closed
for 24 hours, after which the precipitate is collected, washed with hot water
containing a few drops of .ammonium sulphide and treated on the filter
with a mixture of 5 parts of saturated hydrogen sulphide solution and i
part -of hydrochloric acid (D 1-2), The iron, zinc -and manganese sulphides
•are dissolved and these metals are determined as in (b) and (c] ; nickel
and-cobalt sulphides remain on the filter and are determined as in (a}.

cobalt sulphides, which have remained undissolved, are ignited and weighed
as oxides, which are then dissolved in aqua regia, the solution rendered
alkaline with ammonia, and the nickel precipitated with dimethylglyoxime
(see Argentan).

(b)  DETERMINATION OF THE IRON.   The hydrogen sulphide is expelled
by boiling from the hydrochloric acid solution containing the iron, zinc
and manganese, oxidation being effected by a few drops of nitric acid and
the liquid made alkaline with ammonia.   The precipitated ferric hydroxide
is collected, washed and dissolved in hydrochloric acid, the iron being then
reprecipitated with ammonia and weighed as ferric oxide :  Fe203 X 0-6994
= Fe.

(c)  DETERMINATION OF THE ZINC.   The filtrates from the iron precipi-
tate are together rendered alkaline with ammonia and treated in a flask
with 2-3 c.c. of ammonium sulphide, the flask being then filled with water,
stoppered and left for 24 hours.   The zinc and manganese sulphides are
then collected, washed and treated on the filter with dilute acetic acid,
which dissolves only the manganese sulphide.   The residue, consisting of
zinc sulphide, is dissolved on the filter by dilute hydrochloric acid and the
solution evaporated to dryness in a tared crucible.   The residue is mixed
with an aqueous suspension of a little pure mercuric oxide free from alkali,
evaporated to dryness, heated gently over a naked flame to eliminate the
mercury, ignited over a blowpipe flame and weighed : ZnO X 0-8034 = Zn.

(d)  DETERMINATION OF THE MANGANESE.   The acetic acid solution of
the manganese is concentrated and treated with ammonia and hydrogen
peroxide to precipitate the manganese, the precipitate being collected,
washed, dried, ignited and weighed:   Mn304 X 07203 = Mn.

3. Determination of the Arsenic and Antimony.—(a) DETERMI-
NATION OF THE ARSENIC. The alkaline sulphide solution obtained as
described on p. 245 is acidified with acetic acid and heated for '3-4 hours
on the water-bath, the arsenic and antimony sulphides and sulphur which
separate being filtered off. The precipitate is washed with saturated
hydrogen sulphide solution slightly acidified with acetic acid, dried, freed
from sulphur by means of carbon disulphide, filtered, and the arsenic and
antimony sulphides dissolved in hydrochloric acid and potassium chlorate.
The liquid is filtered through a small filter, and the filtrate treated with
0-5 gram of tartaric acid and neutralised with ammonia; the solution,
which should occupy about 20 c.c., is then treated with 10 c.c. of concen- the elec-